Common to prior art magnetic type article surveillance systems is the detection of perturbations induced in an incident magnetic field by an article marker in the course of reversal of magnetic polarity of the field. Typically, such prior art systems include a magnetic field generator, operative to establish an alternating magnetic field in an area of interest, i.e., a surveillance control zone, and a receiver operative to detect perturbations in the magnetic field which may be induced, specifically those produced by such markers.
When the marker magnetic material is driven around its hysteresis loop, from one polarity to the opposite, as occurs upon its exposure to the alternating magnetic field, a signal pulse is produced by the receiver. The shape of this pulse is a function of the time it takes to reverse polarity, i.e., proceed from one saturation point to the other, or from a residual induction point to the reverse saturation point. This time element, in prior art systems, is a function of the time rate of change of the incident field between levels sufficient to effect such polarity reversal.
The primary prior art effort has been directed to the finding of marker magnetic materials with higher and higher permeability and lower and lower coercivity, thereby to give rise to increased slope of the transition from one polarity to the other, otherwise stated, lesser time for the transition. Since the generation of higher order harmonics of sufficient amplitude to be readily detectable attends such increased slope, enhanced discrimination as against perturbations induced in the magnetic field by commonplace objects in the surveillance control zone is thereby attainable. With the same purpose in view, prior art systems have looked to operation at relatively high frequencies and/or with strong incident fields, and the latter is generally sought by establishing narrow surveillance control zones to limit the distance from marker to antenna.
In applicant's view, these efforts have not yielded magnetic markers which produce article tags which, in response to a surveillance field interrogation, provide a signal sufficiently distinguishable that the marker is free from being mimicked by at least some commonplace article. For example, certains samples of nickel plating have been observed to produce signals, responsive to such magnetic fields, that cause false alarms in systems intended to selectively respond to markers containing Permalloy as their magnetic matter.
In the above-referenced related '005 patent application of applicant, incorporated herein by this reference thereto, applicant reports the inclusion, in magnetic tag markers, of a magnetic material exhibiting a reversal of magnetic polarity that occurs in a regenerative fashion, such as with a large Barkhausen discontinuity in its hysteresis loop.
In a specific embodiment, the marker of the referenced related '005 application comprises a body of magnetic material having a magnetic hysteresis loop with a large Barkhausen discontinuity such that exposure of the body to an external magnetic field, whose field strength in the direction opposing the instantaneous magnetic polarization of the body exceeds a predetermined threshold value, results in a regenerative reversal of the magnetic polarization. Quite high harmonics of readily detectable amplitude are provided by the marker, as shown and discussed in the related application.
The related '005 application notes, at page 18 thereof, that amorphous metal wire, obtained directly from the rapid quench of molten metal, evidences the hysteresis loop desired and above discussed. The referenced text notes further that the annealing of such wire gives rise to the loss in such metal wire of its magnetic discontinuities.
In one prior art magnetic type system, deactivation of a magnetic marker is effected by the inclusion in a marker of first and second separate and distinct components of diverse magnetic material, the first serving to generate the detectable signal, and the second serving, upon the occurrence of certain marker deactivating events, to mask and render inoperative the first component. Such masking takes place at a deactivation station and is effected by subjecting the composite marker to a magnetic field of such strength as to activate the second component.
Typically, the marker is subject to a magnetic field adapted to provide output indication of an alarm condition upon presence of the marker in the surveillance zone on the basis of magnetic polarity reversal of the first marker component. On the other hand, upon the presence of the article with marker in an authorized checkout area preceding the surveillance zone, one can deactivate the marker by disposing the same in a magnetic field of character activating the second component that in turn changes the magnetic response of the first marker component.
Another prior approach to marker deactivation involves the formation, in a resonant frequency marker printed circuit, of a fusible link, i.e., a portion of lessened cross-section than the remaining marker printed circuitry, and the disrupting of the link by exposing the marker to increased field energy sufficient to disrupt the integrity of the link. Whereas the marker was of resonant frequency for alarm activation prior to the link disruption, it becomes otherwise upon that event, and passes freely through the surveillance control zone.
The deactivation schemes of the referenced prior art have evident disadvantages: the former in its requirement for plural separate components, respectively for activation and deactivation of the marker, and the latter in its requirement for fusible link formation in the marker printed circuit.
The present invention has as its primary object the provision of an improved system, method and apparatus for the detection of unauthorized marker presence in a surveillance control zone and deactivation thereof at locations preceding entry into such control zone.
A more particular object of the invention is to provide for improved deactivation method and apparatus for magnetic markers in article surveillance systems.
In attaining the foregoing and other objects, the invention provides, in its product aspect, an electronic surveillance system marker which may comprise a unitary active component responsive to incident magnetic energy for causing an associated article surveillance system to render an output alarm, the marker being adapted to be deactivated through change in the molecular organization of the active component, without requiring disruption of the component or change in its chemical composition.
In its method aspect, the invention provides for deactivating an article surveillance marker such as of type having an active component responsive to incident magnetic energy for causing an associated article surveillance system to render an output alarm, the method including a step of modifying the molecular organization of the active component.
In a further aspect, the invention provides an electronic article surveillance system operative with an article marker such as of type comprising a component responsive to incident magnetic energy for causing an associated article surveillance system to render an output alarm, the marker being adapted to be deactivated through change in the molecular organization of its active component, such system comprising transmitting means for establishing an alternating magnetic field in a control zone of interest, receiving means for detection in said control zone of the presence of such marker if same is not deactivated, and means for deactivating such marker through such molecular organizational change.
Turning more particularly to the preferred products, methods and systems of the invention, the marker active component is selected to be of molecularly unorganized, e.g., amorphous matter, provided such as by metal wire obtained directly from the rapid quench of molten metal and having dimensions below discussed. In one product aspect, the marker is used in such unannealed state as a surveillance device. The deactivation step involves molecularly organizing such matter, e.g., by rendering crystalline at least a portion of the component. Such deactivation step is desirably practiced by maintaining such portion of the marker component at a temperature above the crystallization temperature of the component and thereby to crystallize a coercive force in that portion different from its previous coercive force.
In a preferred embodiment, the marker deactivating means of systems of the invention modifies the molecular organization of the marker component by including an electric current supply for selective electrical connection to at least a portion of the marker component and providing such current level therein as to maintain the portion of the marker component at a temperature above the crystallization temperature of the component, thereby to crystallize such coercive force in the portion different from its previous coercive force. Radiant energy may also be employed in this deactivating practice.
Alternatively, the marker active component has stress mechanically induced therein, as by annealing wire in twisted state and constraining same in untwisted form following cooling. Stress-relieving deactivation here involves the relieving of such retained mechanical stress, as by releasing the constraint on the active component. In this instance, the deactivating means may impart mechanical force or radiant energy to the marker component.
The foregoing and other objects and features of the invention will be further understood from the following detailed discussion of preferred embodiments and practices thereof and from the drawings wherein like reference numerals identify like parts throughout.